Streptococcus pneumoniae is the leading cause of community-acquired pneumonia, meningitis, and otitis media in the United States (Brown et al., 1998). While traditional antimicrobial therapy has proven an effective treatment in the past, the emergence of penicillin- and multidrug-resistant strains has resulted in an increasing number of cases of illnesses and fatalities (Doern et al., 1999; Jacoby, 1996). Pneumococcal isolation and identification are complicated by antimicrobial suppression of growth in culture and contamination by normal flora alpha-streptococci. Detection by classical techniques, culture, and serological methods can be time-consuming and indeterminate. Sensitive and specific assays that can be completed quickly in the clinical laboratory are essential for early diagnosis and effective therapy. Molecular assays are inherently valuable because detection can be achieved with enhanced sensitivity and specificity, and detection is not diminished with nonviable organisms. Various molecular methods have been employed to assist investigations (Gillespie, 1999; Hall, 1998; Olive and Bean, 1999).
Accurate pneumococcal disease diagnosis has been frequently hampered not only by the difficulties in obtaining isolates of the organism from patient specimens, but also by the misidentification of Pneumococcus-like viridans streptococci species (P-LVS) as Streptococcus pneumoniae (Spn). This is especially critical when the considered specimen comes from respiratory site.
A major area of focus in pneumococcal disease research has been in vaccine development. The failure of the licensed 23-valent polysaccharide vaccine to provide protection in young children (<2 years of age), the elderly, or the immunocompromised (Forrester et al., 1987) led to development of a second-generation protein-conjugate vaccine, soon to be licensed. This vaccine, composed of the seven most frequent invasive disease-causing capsular serotypes, may overcome the problems of poor immunogenicity associated with the 23-valent vaccine. However, there are indications that this protein-conjugate vaccine may not prevent replacement carriage of serotypes not contained in the vaccine (Obaro et al., 1996). These concerns, along with reports of an increase in antibiotic-resistant pneumococci (Centers for Disease Control and Prevention, 1997), have shifted interest towards the development of a vaccine based on immunogenic pneumococcal species-common proteins of S. pneumoniae (Hammerschmidt et al., 1997). The most promising of these proteins include pneumolysin (Paton, 1996), pneumococcal surface protein (PspA) (Briles et al., 1988), and of particular focus in this study, pneumococcal surface adhesin A (PsaA) (Sampson et al., 1994).
PsaA, encoded by the psaA gene, is a 37-kDa surface protein first identified by Russell et al. (Russell et al., 1990). Monoclonal antibody studies suggest that PsaA is expressed in all 90 serotypes of S. pneumoniae (Crook et al., 1998), and PCR-restriction fragment length polymorphism analysis of the 23 vaccine serotypes demonstrated the conservation of the psaA gene (Sampson et al., 1997).